In an imaging device using a solid-state image sensing device, an optical filter that transmits light in a visible range (hereinafter, referred to also as “visible light”) and blocks light in a near-infrared range (hereinafter, referred to also as “near-infrared light”) is used in order to reproduce the color tone satisfactorily and obtain a clear image. As the optical filter, a near-infrared cut filter including an absorption layer containing a near-infrared absorbing dye and a reflection layer composed of a dielectric multilayer film blocking light in an ultraviolet range (hereinafter, referred to also as “ultraviolet light”) and the near-infrared light is known (refer to Patent Reference 1 (JP-A 2013-190553), Patent Reference 2 (JP-A 2014-052482), Patent Reference 3 (International Publication WO2014/002864). More specifically, the dielectric multilayer film itself changes in spectral transmittance curve depending on an incidence angle, and the near-infrared cut filter including both of the reflection layer and the absorption layer can obtain a spectral transmittance curve with the incidence angle dependence suppressed by the absorption characteristics of the absorption layer.
The absorption layer is obtained, for example, by dispersing a near-infrared absorbing dye exhibiting maximum absorption in wavelengths of 700 to 750 nm and having a steep absorbing curve of light in wavelengths of 630 to 700 nm in a transparent resin. Here, when the content of the dye is reduced to prevent the absorption layer from causing a decrease in the transmittance for light of wavelengths of 440 to 600 nm where the visibility is high, a near-infrared absorption band where a predetermined absorptance is obtained narrows. In this case, the incidence angle dependence of the reflection layer becomes obvious so that a wavelength range where light blocking is insufficient may appear.
The optical filters in Patent References 1 and 2 generally obtain an effect of decreasing the incidence angle dependence of the transmittance by the absorption layer in a wavelength range where the transmittance becomes more than 50% in a transmittance transition range where the transmittance greatly changes, of wavelengths of 600 to 700 nm. However, in a wavelength range where the transmittance is 50% or less in the transmittance transition range, even the absorption layer cannot sufficiently absorb light, causing a problem that the incidence angle dependence of the spectral transmittance in the reflection layer becomes obvious as the incidence angle dependence of the optical filter.
Hence, to expand the absorption band suppressing the incidence angle dependence, an absorption layer containing two or more kinds of near-infrared absorbing dyes different in maximum absorption wavelength is under discussion. Patent References 2 and 3 disclose optical filters for which two or more kinds of near-infrared absorbing dyes different in maximum absorption wavelength can be used. Besides, Patent Reference 4 (International Publication WO2012/169447) discloses an optical filter using an absorption layer containing two kinds of near-infrared absorbing dyes having different maximum absorption wavelengths such as wavelengths of 695 to 720 nm and wavelengths of 720 to 800 nm. Further, Patent Reference 5 (Korean Registered Patent 1453469) discloses an optical filter using an absorption layer containing two kinds of light absorbents having different maximum absorption wavelengths such as a wavelength of 685 nm and a wavelength of 702 nm.
The optical filter including an absorption layer containing two kinds of near-infrared absorbing dyes as described above is easy to suppress the incidence angle dependence of the transmittance in the transmittance transition range. However, if the absorbency for the near-infrared light is tried to increase by increasing the additive amount of the dyes, an absorption band where merely slight light absorption can be recognized broadens, so that the transmittance for visible light where high transmittance should be normally obtained also decreases. On the other hand, if the additive amount of the dyes is decreased, the performance of blocking the near-infrared light by absorption decreases, so that the above-described incidence angle dependence becomes obvious. As a result, the suppress of incidence angle dependence and the securement of high visible light transmittance are in a trade-off relation, causing a problem that an optical filter capable of obtaining both characteristics cannot be realized.